This invention relates to a jamming cancellation system and more particularly to a multiple source jamming cancellation system.
An important consideration in the design of radar systems, particularly those for military applications, is the provision of means for recovering the radar signal from a total signal which includes noise jamming signals of higher intensity than that of the radar signal itself. Most noise jammers consist of an oscillator which is frequency modulated by a noisy waveform at a high rate. The rates are high enough to shock excite the victim radar i.f. amplifier so that its output is indistinguishable from true random noise.
Various techniques to cancel jamming signal and thus recover the desired signal have been proposed. For example, patent applications for three such jamming signal cancellation systems have been filed: Ser. No. 509,158, filed Sep. 24, 1974 now U.S. Pat. No. 5,363,104; Ser. No. 446,570, filed Feb. 21, 1974 abandoned and Ser. No. 491,587, filed Jul. 22, 1974 pending, all assigned to the assignee of this application. For illustration purposes one of these applications will be considered, namely Ser. No. 509,158.
In that application improved desired signal detection is achieved by detecting the AM difference frequency produced by the beating of the desired signal with the jamming signal and applying same to one input of a balanced modulator while simultaneously applying the received signal (containing both the desired signal and the jamming signal) to the other input of the balanced modulator whereby the output from the balanced modulator contains the desired signal but not the jamming signal. Actually, the video signal applied to the balanced modulator causes double-sideband suppressed-carrier modulation of the jamming signal. As a result the balanced modulator output contains two frequencies: one is the desired signal; the other is an image on the other side of the jamming frequency and frequency modulated with twice the deviation of the jammer. The image is outside the passband of the radar receiver and thus, only the desired signal is processed.
This single stage device and those of the other aforementioned patent applications only remove the strongest jammer signal. If a weaker jamming signal is also present, it is preserved in its original ratio to the desired signal. Therefore, the apparent improvement factor can never exceed the ratio of the two jammer powers and it generally runs a few dB less.
To solve this problem of multiple jammers, one can, in principle, cascade two of such cancellation systems, let the first one take out the stronger jammer and let the second one take out the weaker jammer, leaving the desired signal. In reality the solution is somewhat more complex since, as mentioned, the cancellation system produces an image of every signal at the output of the first cancellation system, therefore the weaker jammer signal is accompanied by its image. Applying this to the second cancellation system would be equivalent to applying two jammers of equal strength and the second cancellation system cannot cope with this problem.